A computer network is a means of exchanging or transferring information (e.g. data, voice, text, video, etc.) between host machines in the network. The network comprises host machines connected by a communication subnet. The subnet comprises nodes (also called switching elements) connected, to each other and to the hosts, by links. The information, often conveniently formatted into packets or cells, is transferred between a source host (also called a "source") and one or more receiving hosts (also called "destinations" or "endpoints ") on a path by selecting a set of links and nodes in the communication subnet to form the path. See, Andrew S. Tanenbaum, Computer Networks, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1981.
There are three types of designs for the communication subnet: 1) point-to-point subnets, 2) broadcast subnets and 3) multicast subnets. In a point-to-point subnet, if two hosts wish to communicate, but do not have a single link directly connecting them, then the hosts must communicate indirectly, i.e. via intermediate nodes. Typically, a communication is received in its entirety at each intermediate node, stored in buffers in the node until the next required link in the path is free, and then forwarded. In a broadcast subnet, information sent by any host is received by all other hosts. The information typically contains an address specifying the host for which the information is intended. Upon receiving the information, each host checks the address, and if the information is intended for that host, it is processed; otherwise the information is ignored. In a multicast subnet, a communication is received by a subset of hosts in the network. Multicasting may be point-to-multipoint (where a single host transmits to a plurality of hosts, e.g. electronic distribution of books from a publisher to bookstores across the nation) or multipoint-to-multipoint (where a subset of hosts in a network transmits information among the hosts in the subset, e.g. nationwide video conferencing).
A multicast protocol is a set of rules for conveying information from a single host to multiple hosts in a network or for conveying information among a subset of hosts. Although multicast protocols have been designed for unreliable broadcast network and satellite broadcast channels, (see, J. M. Chang and N. F. Maxemchuk, "Reliable Broadcast Protocols," ACM Trans. Comp. Sys., Vol. 2, No. 3, 251-273, August 1984; K. Sabnani and M. Schwartz, "Multidestination Protocols for Satellite Broadcast Channels," IEEE Trans. Comm., Vol. COM-33, No. 3, March 1985) the design of multicast protocols in general, and of multicast protocols for wide area, broadband, high speed networks in particular, presents special challenges. First, since each host must acknowledge information received, the protocol must overcome the acknowledgement implosion problem (i.e. the receipt at the source host of multiple acknowledgment signals) inherent in any multicasting scheme. Second, propagation delays (due to the physical area served by the network and the finite speed of signals transmitted over the network) combined with high transmission rates (due to a desire to transmit as much information as possible in the shortest time possible) produce large delay-bandwidth products.
Large delay-bandwidth products tend to increase the need to retransmit (i.e. remulticast) information. This is because: 1) at any given time a large amount of information is on the network (due to the large network bandwidth), and this large amount of information must be remulticast when information is lost--e.g. due to overflowing buffers in nodes in the network, and 2) feedback or acknowledgment signals from receiving hosts take a non-zero amount of time to reach the source host (due to propagation delays) thereby causing the source host to retransmit due to an absence of, or delay in, acknowledgments.
It is desirable that a multicasting source avoid unnecessary remulticasts since this will further increase the end-to-end delay of the protocol and increase traffic on the network. Hence, there is a need for a multicast protocol for broadband networks that can avoid unnecessary retransmissions of information and avoid the acknowledgment implosion problem while advantageously providing high throughput and low delay in information transmission.